Glucose is a basic fuel of the human body (as well as of many other organisms) and is delivered throughout the body through the blood. The rate of glucose production, also referred to as glucose rate of appearance and glucose Ra, is about 2-3 mg/min/kg of body weight in a healthy person while at rest, and can be as high as 8 mg/min/kg or more under stress such as exercise or illness. Pyruvate and lactate, which are both gluconeogenic precursors and products of glucose catabolism, are also basic fuels of the human body and other organisms.
Glucose, a six-carbon (hexose) sugar, is an essential fuel energy source for several vitally important organs and tissues in the body, including the brain and nerves that require a continuous glucose supply, including after injury. Not surprisingly, glucose is an important and tightly regulated metabolite.
Glucose Ra should not be confused with blood concentration of glucose, also called [glucose]. The latter is a simple measure of the total amount of glucose in the blood, as opposed to the rate of production. The [glucose] is a common measurement taken from a blood samples, as in standard doctor office visits and home diabetes diagnostics. This value can vary significantly in resting individuals, but generally averages about 90-100 mg/dl blood or 5.5 mM. Physiologically, glucose can appear in the blood of a person by three major means: delivery from ingested carbohydrate-containing foods, hepatic glycogenolysis (“GLY”), and gluconeogenesis (“GNG”) (hepatic and renal). The recommended dietary allowance for carbohydrate-containing foods is about 130 g/day, a value determined to be the minimal daily brain glucose requirement (8) (note that non-patent literature citations are made as numbers in parentheses, and the corresponding references are listed at the end of this specification). Hence, dietary carbohydrate and total nutrient inadequacy will reflexively cause increased GLY and GNG to maintain glucose requirements for the brain, other tissues with high glucose needs (nerves, red blood cells, kidneys) and the body in general.
Glucose production occurs by GLY and GNG. It is generally better if the majority of glucose production is from GLY. This is because GLY is an efficient process of glucose production, in that it is simple breakdown of glycogen, a glucose polymer stored mainly in the muscles, liver and kidneys. Normally, at rest, in a nourished state, most glucose is produced by GLY (typically over 75%). This number can decrease under stress such as exercise or illness, as the body needs to produce more glucose than can be provided by GLY.
Gluconeogenesis (“GNG”) describes essentially all pathways for producing glucose other than glycogenolysis (“GLY”). GNG produces glucose from carbon substrates such as pyruvate, lactate, glycerol, and gluconeogenic amino acids, among others. These can be termed GNG precursors. GNG is less efficient than GLY in terms of glucose produced per unit of stored energy because of the more complex pathways needed to produce it. Since it is less efficient than GLY, it is generally not preferred by the body, but can be used to produce glucose as needed. GNG is less efficient than GLY in other ways as well. The work of raising a GNG precursor to the level of glucose 6-phosphate and glucose requires significant energy input, and important body constituents such as lean body mass, muscle are often degraded to provide precursor materials for the process. GNG also may be used to access glycogen stored elsewhere in the body instead of direct conversion of that glycogen to glucose.
The current art in the measurement of metabolic state and treatment has at least two significant categories of problems. One is that no biomarker measurements, either alone or in combination, are used in the current art to give an accurate picture of the overall BES of a patient. To the degree that measurements are made in the current art, such as with [glucose], they are inadequate indicators of the BES.
The biomarker [glucose], is well known in the art and simple to assess from a blood test. While a large shift (either low or high) in [glucose] can be cause for concern and inform the type of feeding the patient receives, it does not provide a good indicator of the BES of a patient, especially within its typical ranges. Indeed, the maintenance of blood glucose homeostasis is a top physiological priority, and there are diverse and redundant body mechanisms to maintain blood [glucose]. Thus a normal [glucose] may belie metabolic stresses that are going on, with the body working very hard to maintain [glucose]. Among those mechanisms are GNG, a critically important process about which the blood [glucose] measurement provides no direct information.
Another biomarker, glucose rate of appearance (“Ra”), gives only a slightly better indicator of the BES of the patient. A high glucose Ra, for example, indicates that the patient may be experiencing a stress (such as injury, exercise or starvation) that has induced a high glucose production. While this is a somewhat useful, there is need for a biomarker that is a more precise indicator of BES. In addition, determination of glucose Ra is complex, time consuming and costly. It requires labeled glucose to be given to the patient, typically glucose with deuterium (typically noted as simply D or 2H as opposed to merely H, hydrogen), or carbon 13 (13C), and comparison of labeled and non-labeled glucose (the latter produced by the glucose pathways) to determine Ra (80).
The complex, costly and time-consuming process of determining glucose Ra with stable isotopes of H (typically deuterium) or 13C-glucose is well described in the literature (2, 26, 55). It is typically done as follows. Control subjects or patients receive a primed continuous infusion of [6,6−2H]glucose, i.e., D2-glucose, glucose with two deuteriums on carbon number 6 (C-6) diluted in 0.9% sterile saline and tested for pyrogenicity and sterility prior to infusion. To hasten achievement of a constant blood isotopic enrichment, a priming bolus of perhaps about 125 times the continuous per minute infusion rate, or about 250 mg D2-glucose, is infused over several min prior to commencement of a continuous tracer infusion of 2.0 mg·min−1D2-glucose. In this manner, isotopic equilibration in the blood can be achieved in 60-90 min (about half the time to isotopic equilibration in blood if a priming tracer dose is not given).
To verify when isotopic equilibration has been achieved, several ml of blood is drawn serially. Verification can be done by mixing in several volumes of 6-8% perchloric acid (“PCA”), and the deproteinized supernatant analyzed by means of forming a penta-acetate derivative followed by analysis using gas chromatography/mass spectrometry (“GC/MS”).
For simultaneous concentration analysis, known amounts of a labeled internal standard, such as uniformly labeled glucose, where each carbon of the glucose is labeled, by for example, the carbon 13 isotope, thus noted [U-13C]glucose, is used. The glucose molecule thus has an increased mass of about 6 atomic units (“au”) (m+6). This labeled glucose is added to the supernatant of control subject or patient samples collected in perchloric acid. To separate glucose, samples are neutralized with 2N KOH and transferred to cation resin, ion exchange columns such as 50W-X8 (from Bio-Rad Laboratories). Glucose is eluted first with doubly deionized H2O (the anions, and cations, by contrast, are retained on the column).
The glucose ion-exchange effluent is reduced by lyophilization and derivatized by resuspending the lyophilized sample in a small amount (e.g., 1 ml) of methanol, a small amount [e.g., 200 microliter (μl)] is transferred to a 2 ml microreaction vial and dried under N2 gas. A small amount (e.g., 100 μl) of a 2:1 acetic anhydride-pyridine solution is added to each sample vial and heated at 60° C. for 10 min. Samples are again dried under N2 gas, resuspended in a small amount (e.g., 200 μl) of ethyl acetate, and transferred to micro vials for analysis.
Glucose isotopic enrichment (“IE”) is determined by GC/MS, for instance with a GC model 6890 series and MS model 5973N, from Agilent Technologies) of the penta-acetate derivative, where methane is used for selected ion monitoring of mass-to-charge ratios (m/z) 331 (non-labeled glucose), 332 (M+1 isotopomer, [1-13C]glucose), 333 (M+2 isotopomer, D2-glucose), and 337 (M+6 isotopomer, [U-13C]glucose, the internal standard). Whole blood glucose concentration is determined by abundance ratios of 331/337. Selected ion abundances are compared against external standard curves for calculation of concentration and isotopic enrichment.
Therefore there is a need in the art for a biomarker that is a good indicator, by itself, of BES, as well as simple and effective methods of estimating that biomarker.